The Local Area Augmentation System (LAAS) is in the late stages of being developed to support the differential Global Navigational Satellite System (GNSS) based aircraft precision approaches and landings. Applications other than precision approach and landing may also be supported. The LAAS, when implemented using the Global Positioning System (GPS) as the source of satellite navigation signals, is known as the GPS/LAAS and is shown in FIG. 17. It consists of three primary subsystems:                A) The satellite subsystem, which produces ranging signals. This standard explicitly addresses the use of GPS and SBAS (satellite based augmentation system). Provision has been made for the use of other satellite systems such as the Russian GLONASS.        B) The ground subsystem, which provides a VHF data broadcast (VDB) containing differential corrections and other pertinent information. Ground-based ranging signals may also be provided by airport pseudolites (APL's) to enhance system availability.        C) The airborne subsystem, which encompasses the use of aircraft equipment, receives and processes the LAAS/GPS signal in space in order to compute and output a position solution, deviations relative to a desired reference path and appropriate annunciation.        
First, the GPS satellites provide both the airborne subsystem and a ground-based subsystem with ranging signals. Second, the ground subsystem produces ground-monitored differential corrections and integrity-related information as well as data including the definition of the final approach path, a geometric path in space to which the aircraft on approach will navigate. These data are transmitted on a VHF data broadcast (VDB) to the airborne subsystem. The content and format of the data provided via the VDB are defined in RTCA, GNSS-Based Precision Approach Local Area Augmentation System (LAAS) Signal-in-Space Interface Control Document (ICD). Third, ground based APL's may be used to provide additional ground-monitored ranging signals to the airborne system.
The airborne subsystem uses the GPS/LAAS SIS (signal in space) to calculate a differentially-corrected position estimate and generates deviation signals with respect to the final approach path. The airborne subsystem also provides appropriate annunciations of system performance (e.g alerts). A position-velocity-time (PVT) output with integrity is also provided and may support other applications.
The airborne subsystem outputs are formatted as appropriate to interface with other aircraft equipment used to support the particular operation. For example, “ILS lookalike” deviation outputs are provided to aircraft displays and/or navigation systems. The airborne subsystem also provides appropriate annunciations of system performance (e.g alerts).
A plurality of IMLA's (FIG. 1) described herein, significantly reduce the inherent multipath corruption in ground based GPS reference stations. This multipath reduction ability allows the LAAS to support CAT I/II/III type approach and landings with the accuracy and integrity required while only depending on the GPS L1 C/A carrier smoothed code signal.
Published research on methods to reduce the amount of ground multipath by Ohio University have helped optimize the method used to attenuate the LAAS GPS multipath to a minimum. As shown in FIG. 18, Ohio University's idea of dividing the hemispherical coverage into 2 separate antenna beams has allowed an antenna design which can provide significantly better multipath performance than a single beam approach. This concept was originally published in 1994 by Ohio University.
This type of dual beam antenna system divides the required hemispherical coverage volume into two or more pieces in order to optimize the antenna's performance. The main discovery that Ohio University made when proposing this dual beam antenna approach was that the High Zenith Antenna could be made to fill in the natural “Null” that occurs directly above a collinear array of vertically polarized radiating elements.
The dominant error source in differential global positioning systems (DGPS) applications is multipath. Multipath occurs when a signal arrives at its destination via multiple paths resulting from reflections and/or diffractions. Multipath is troublesome to navigation ranging systems when the signal amplitude of the multipath is strong relative to the direct signal. In addition, since reflections and diffractions involve larger path lengths than the direct signal, they incur a time delay, which can affect GPS code or carrier measurements. This time delay is a significant problem for GPS since it performs time-based ranging measurements.
Ground multipath from satellite transmissions is the largest error source for the LAAS because of its close proximity to the ground and its nearly static geometry.
The invention described herein deals mainly with antenna techniques to reduce ground multipath. By shaping the antenna gain, phase and group delay patterns appropriately, the amount of multipath, phase and group delay errors that enter the receiver front end can be significantly reduced. A common way to characterize an antenna's multipath rejection capability is in terms of a power ratio referred to as the desired-to-undesired (D/U) ratio. The D/U ratio is also known as direct-to-indirect ratio, down-to-up ratio and a variety of other names. The D/U ratio is calculated for a given elevation angle in order to assess the ground multipath rejection capability of an antenna and thus tells how many dB of multipath can be rejected after the radio frequency (RF) stages of a transmitter and before the RF stages of a receiver. D/U is shown graphically in FIG. 19.